Residual petroleum oil fractions produced by atmospheric or vacuum distillation of crude petroleum are characterized by a relatively high metals content. This occurs because substantially all of the metals present in the original crude remain in the residual fraction. Principal metal contaminants are nickel and vanadium, with iron and small amounts of copper sometimes being present.
The high metals content of the residual fractions generally preclude their effective use as chargestocks for subsequent catalytic processing, such as catalytic cracking and hydrocracking, because the metal contaminants deposit on the special catalysts for these processes and cause the formation of inordinate amounts of coke, dry gas, and hydrogen.
It is current practice to upgrade certain residual fractions by a pyrolytic operation known as coking. In this operation the residuum is destructively distilled to produce distillates of low metals content and leave behind a solid coke fraction that contains most of the metals. Coking is typically carried out in a reactor or drum operated at about 800.degree.-1100.degree. F. temperature and a pressure of 1-10 atmospheres. The economic value of the coke byproduct is determined by its quality, particularly its sulfur and metals content. Excessively high levels of these contaminants make the coke useful only as low-valued fuel. In contrast, cokes of low metals content, for example up to about 100 ppm (parts per million by weight) of nickel and vanadium and containing less than about 2 weight percent sulfur, may be used in high-valued metallurgical, electrical, and mechanical applications.
Presently, catalytic cracking is generally accomplished by utilizing hydrocarbon chargestocks lighter than residual fractions which usually have an API gravity greater than 20. Typical cracking chargestocks are coker and/or crude unit gas oils, vacuum tower overhead, and the like, the feedstock having an API gravity from about 15 to about 45. Since these cracking chargestocks are distillates, they do not contain significant proportions of the large molecules in which the metals are concentrated. Such catalytic cracking is commonly carried out in a reactor operated at a temperature of about 800.degree.-1500.degree. F., a pressure of about 1-5 atmospheres, and a space velocity of about 1-100 WHSV.
The amount of metals present in a given hydrocarbon stream is often expressed as a chargestock's "metals factor". This factor is equal to the sum of the metals concentrations, in parts per million, of iron and vanadium plus ten times the concentration of nickel and copper in parts per million, and is expressed in equation form as follows: EQU F.sub.m =Fe+V=10(Ni+Cu)
Conventionally, a chargestock having a metals factor of 2.5 or less is considered particularly suitable for catalytic cracking. Nonetheless, streams with a metals factors of 2.5-25, or even 2.5-50, may be used to blend with or as all of the feedstock to a catalytic cracker, since chargestocks with metals factors greater than 2.5 in some circumstances may be used to advantage, for instance with the newer fluid cracking techniques.
In any case, the residual fractions of typical crudes will require treatment to reduce the metals factor. As an example, a typical Kuwait crude, considered of average metals content, has a metals factor of about 75 to about 100. As almost all of the metals are combined with the residual fraction of a crude stock, it is clear that at least about 80 percent of the metals and preferably at least 90 percent needs to be removed to produce fractions, having a metals factor of about 2.5-50, that are suitable for cracking chargestocks.
The economic and environmental factors relating to upgrading of petroleum residual oils and other heavy hydrocarbon feedstocks have encouraged efforts to provide improved processing technology, as exemplified by the disclosures of various U.S. Pat. Nos. which include 3,696,027; 3,730,879; 3,775,303; 3,876,530; 3,882,049; 3,897,329; 3,905,893; 3,901,792; 3,964,995; 3,985,643; 4,016,067, and the like.
Efforts have been made in the past to upgrade petroleum residual oils in the presence of solids. For example, U.S. Pat. No. 3,893,911 teaches the demetallization of residua by ebulliated bed catalytic hydrogenation in the presence of particulate activated porous aluminum oxide catalyst. As another example, inert particulate solids, including diatomaceous silica in the form of extruded pellets, are contacted by residua in the presence of hydrogen at 500.degree.-850.degree. F. and at 300-3,000 psig for removing metalliferous contaminants according to the process of U.S. Pat. No. 3,947,347, but the solids must have an average pore diameter of 1,000-10,000 A.
U.S. Pat. No. 4,259,178 teaches the delayed coking of a slurry mixture of a petroleum resid and 10-30 weight percent of caking or non-caking coal, blended at a temperature below 50.degree. C. to produce a soft, porous, fusible, sponge-like cake.
Coking has long been the most important process for upgrading of resid. Because of worsening of crude quality and improvements in vacuum distillation and catalytic cracking technologies, the quality of coker feed has been deteriorating for years. At the present time, the low quality coke produced by some refineries has become difficult to market.
The important quality parameters of coke are metal and sulfur contents and physical structure, namely, shot coke. The high metal and sulfur contents make the coke not only unsuitable as high-value electrode coke but also as low-value fuel because the metals, particularly vanadium, cause boiler tube corrosion. In addition, the sulfur forms SOx and pollutes the air, and the shot coke creates difficulties in pulverization. The need for processes to produce high quality coke is consequently obvious.
Accordingly, it is a main object of this invention to provide a process for production of high quality and marketable coke, having low contents of metals and sulfur, from high metal and sulfur resids.
Another object is to recover the metal values of resids.
A further object is to minimize the environmental effects in production and utilization of coke.
Other objects and advantages of the present invention shall become apparent from the accompanying description and illustrated data.